The present invention relates generally to Selective Catalytic Reduction (SCR) catalysts and, more particularly, to methods and systems for controlling reductant levels in SCR catalysts.
Selective catalytic reduction is an important tool in efforts to meet increasingly strict engine emissions standards. Certain techniques for reducing CO emissions result in greater production of nitrogen oxides, also referred to as NOx. SCR is a means of converting NOx with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant.
A controlled level of NH3 storage buffer in the catalyst is desired in order to maintain high NOx conversion efficiency (μNOx), defined here by
                                          μ            NOx                    =                                                    NOx                ⁡                                  (                  inlet                  )                                            -                              NOx                ⁡                                  (                  outlet                  )                                                                    NOx              ⁡                              (                inlet                )                                                    ,                            (        1        )            where NOx(inlet) is the NOx level proximate an inlet of the SCR catalyst and NOx(outlet) is the NOx level proximate and outlet of the SCR catalyst. As seen in the schematic graph of FIG. 1A, NOx conversion efficiency can be reduced due to too low or too high an amount of stored NH3.
The known technique for controlling NH3 levels is not considered to produce acceptable results. By this technique, a device such as an electronic control unit (ECU) (various suitable devices are hereinafter referred to generically as a controller) estimates the amount of NH3 stored in the SCR catalyst by keeping track of how much NH3 has been added to the system via dosing and estimating how much NH3 has been consumed by reaction with NOx. The first component—addition of NH3—is quite simple because the amount of NH3 added is directly proportional to urea dosing because the urea decomposes to NH3 and CO2 under high temperature conditions with adequate humidity. The second component—consumption—can be somewhat more difficult because it uses an estimated exhaust mass flow in addition to NOx sensor measurements both before and after the SCR to estimate how much NOx is reduced. The technique assumes that the amount of NH3 that is used is directly proportional to the NOx that is reduced.
A problem with the known technique is that error accumulates over time in the stored NH3 calculation, which leads to reduced NOx conversion efficiency. The controller uses the modeled stored NH3 mass as a feedback to a controller that tries to maintain stored NH3 at the target. However with nothing to correct this model over time, there is a risk that the model will diverge from actual NH3 levels. In this case failure to properly control stored NH3 directly leads to reduced NOx conversion efficiency.
The only mechanism to keep the modeled stored NH3 from diverging from actual NH3 levels is to periodically start over by using all up of the NH3 in the SCR and then resetting the model. In addition to having a direct impact on emissions from the time the SCR begins to operate at low efficiency as the actual stored NH3 approaches zero, emissions control can be dramatically compromised if the model diverges from actual levels before the calibration is triggered.
It is desirable to provide a method and a system for controlling NH3 levels to better ensure NH3 levels in an SCR catalyst are kept within a desired range.
In accordance with an aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises measuring a change of NOx conversion efficiency (dμNOx) across the SCR catalyst, measuring a change of reductant level (dB) in the SCR catalyst, comparing a measured ratio dμNOx/dB to a target ratio, and adjusting reductant injection to cause the measured ratio to approach the target ratio.
In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to measure a change of NOx conversion efficiency (dμNOx) across the SCR catalyst, measure a change of reductant level (dB) in the SCR catalyst, compare a measured ratio dμNOx/dB to a target ratio, and control the injector to adjust reductant injection to cause the measured ratio to approach the target ratio.
In accordance with another aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises a) calculating a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, b) determining a first NOx conversion efficiency (μNOx1) at an end of the first period of time, c) changing reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, d) determining a second NOx conversion efficiency (μNOx2) at the end of the second period of time and, if μNOx2>μNOx1, maintaining the second injection rate, and if μNOx2<μNOx1, changing reductant injection by a second change amount
In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to calculate a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, determine a first NOx conversion efficiency (μNOx1) at an end of the first period of time, control the injector to change reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, determine a second NOx conversion efficiency (μNOx2) at the end of the second period of time and, if μNOx2>μNOx1, control the injector to maintain the second injection rate, and if μNOx2<μNOx1, control the injector to change reductant injection by a second change amount in a direction opposite a direction of the change amount.